U.S. Pat. No. 4,770,907 (Kimura, Fuji Paudal, 1988), U.S. Pat. No. 5,143,523 (Matarrese, GE, 1992) and U.S. Pat. No. 5,405,573 (Clark et al., GE, 1995) all describe the use of fluidised beds to coat particles. In this method, ultra hard cores, e.g. diamond seeds, are suspended in a flow of gas within a chamber, into which a fine suspension of binder, solvent and particulate material (e.g. metal powder, the encapsulating material) is sprayed. Alternatively, the binder-solvent may be sprayed with separate powder addition. The emergent pellets are built up in volume proportional (non-linearly) to the residence time spent in the chamber. The advantage of this process is that the fluid bed allows a good separation of the core seeds and thereby ensures that a single core (diamond seed) is contained in each pellet while depositing coating material at a reasonable rate. The disadvantage of this technique is that the maximum deposition rate is relatively slow and when using a high density particulate coating material e.g. Mb, W and WC, and the increasing mass of the pellets presents difficulties in terms of the capabilities of the equipment to maintain the suspension. This can be addressed by increasing the capacity of the equipment but this is costly and impacts on the commercial viability of producing commercial volumes of material. In addition, fluid-bed type coatings are not ideal when coating with a mix of powders of different densities and particle shapes and sizes as the material has a tendency to segregate in the fluid bed and the intended composition may not be uniformly achieved.
The “rotating pan” method as described in GB1014295 (Norton, 1965) and EP0012631 (Tomlinson, DeBeers, 1983) involves introducing the ultra hard core material, e.g. diamond seeds, into either a rotating inclined pan, a drum or any other rotating vessel, where the pellet can be built up by 1) spraying a slurry containing metal powder, binder and solvent (encapsulating material) over the rotating diamond seeds and/or 2) the binder and solvent is/are sprayed separately and the metal powder then “sprinkled” over the rotating diamond seeds. Rotation of the pan separates the coated diamond seeds (emergent pellets) and allows time for removal of the solvent from the sprayed material to form a concentric jacket of encapsulating material which increases in volume as the process proceeds. This technique is efficient in terms of depositing encapsulating material and thus building up the pellet mass quickly. However, the difficulty with this method is that it is susceptible to agglomeration of the cores and/or early pellets in the initial stages of the process. Deposition rates must be very slow to avoid agglomeration and this increases the overall processing time and reduces the throughput of the process. Agglomeration reduces in severity after the emergent pellet has attained a critical size. The consequence of the agglomeration is that the final pellets may have significant size distribution and may contain more than one core per pellet. This contributes to increased process time and cost.
ZA2006/06674 (Flynn et al., E6, 2006) partly over come this problem by adopting a dual stage process for the rapid formation of pellets. Essentially the solution to the problems described above is to combine the two techniques known in the art into a single process design. As such, the initial stages of the process involve a fluid bed approach to maximise the yield of pellets containing one core particle only e.g. diamond seeds. The pellets may be built up to a critical size volume (Vcrit) whilst remaining in a fluid suspension. When the pellets attain this critical size, the pellets are transferred to a rotating pan where the pellets form the (sub) core of the final pellet process. The pellets so produced have a volume significantly greater than the pellets as introduced and the risk of agglomeration is much reduced as the layer on the surface absorbs the spray more quickly and thus deposition rates may be increased. In addition, the weightier particles are less likely to be held together by surface tension of the spray. However, combining both techniques requires two sets of capital equipment and downtime in terms of transferring material from one machine to the other.
Therefore, there is a need to develop a technique which would overcome the technical difficulties outlined above and provide a more cost efficient solution to producing encapsulated abrasives. Preferably such a technique provides an advantage when used in combination with either ‘pan rotating’ or ‘fluidised’ route.
The contents of applicant's South African patent application no. 2007/06077 filed on 23 Jul. 2007 and all applications claiming priority therefrom are herein incorporated by reference.